add a server to handle game events + mirror all traffic to it

firebase realtime db = 💸😭 aws ec2 + elasticache = 😍😇

Infra & Routing

this stuff isn't version controlled at the moment
ec2 instance: server (us-west-1)
elasticache instance: redis (us-west-1)
server & redis are in same vpc
main "website server" proxies traffic to downforacross.com/socket.io to websocket server, and serves static assets otherwise (on downforacross.com/*)

server accepts websocket connections at /socket.io
server writes game events to redis
server stores in-memory state:
- clients (open websocket connections), managed via socket.io
- a map from gids to list of clients subscribed to that game's events
server manages the pub/sub of game events
client talks directly to firebase for all other db transactions (uploading puzzles, creating games, tracking stats)

Messages sent by client:

[on connection]
- nothing happens
join (gid)
- server adds client to list of subscribers of gid, or fails if already joined
sync_all (gid)
- server responds with gid's entire list of events (fetched from redis)
game_event ({gid, event})
- server processes event (replace {.sv: 'timestamp'} with Date.now())
- server adds the event to id's list of events
- server emits a game_event (event) message to all subscribers of gid
[on disconnection]
- server removes client as a subscriber

[x] Send game events to both firebase and server
[x] Receive game events from both firebase and server
[x] Use server game events as source of truth for beta games, use firebase game events by default
[x] Optimistic rendering (when client initiates a game_event message, optimistically anticipate game_event message from the server before it's received)
[x] Read the url param ?ws=1 to determine whether to create games in beta (poor man's feature flagging)